In color reproduction, it is often necessary to verify the color accuracy of separation negatives and/or positives used in the printing process. This can be accomplished by producing a four or more color proof. There are two main types of color proofs: an overlay proof and a surprint proof. An example of a negative acting overlay proof is described in U.S. Pat. No. 3,136,637. That proofing method is based upon exposing and developing each presensitized color sheet separately and then superimposing the sheet to yield a multicolor composite. This method offers a very simple way of combining two or more colors in registration. However, it has several inherent disadvantages. The multiplicity of plastic sheets cause incident light to internally reflect. This reflection imparts a gloss, a color hue shift and a distortion of the image which is not truly representative of printed copy. U.S. Pat. No. 4,634,652 describes an improved overlay system using a more transparent substrate. However, an optical distortion of the image and subsequent color hue shift still remains due to the large spacing between the colored images.
A surprint proof is generated by successively superimposing different colored layers upon each other on a single sheet. The surprint color proofing materials are composed of two primary types: photosensitive precolored sheets and photosensitive colorless sheets whose latent image may be toned with pigments, inks or dyes. Some examples of post-colored proofing materials may be found in U.S. Pat. Nos. 3,060,024, 3,634,087 and DE No. 3,336,431. In these systems the image is weak and often requires a protective layer. The resultant color hue is also very user dependent due to dependency upon color density control and lot to lot variation of the colorants.
Some examples of precolored proofing materials include constructions based upon both diazo and photopolymers as the photosensitive element. In U.S. Pat. Nos. 4,482,625 and 4,304,836 two different precolored photopolymer constructions are described which are imaged prior to lamination to the substrate or intermediate sheet. A similar system is described in GB No. 2,144,867 A which utilizes precolored films sensitized with diazo based polymers. These systems require the imaged films to be laminated in register which is very dependent upon laminating speed, laminating temperature, and dimensional stability of the film and substrate. The accuracy of the registration is limited to small format proofs. This type of system also makes it very difficult to produce several small proofs on a single large sheet (scatter proofing).
U.S. Pat. Nos. 3,671,236, 4,260,673, 4,650,738, 4,656,114, 4,659,642 and EP No. 0,115,899 A3 describe diazo sensitized precolored constructions which are laminated to the substrate prior to imaging. Each of these systems contain barrier and/or adhesive layers which remain in the final proof, lying between each image layer. The existence of these additional clear layers causes an optical enlargement of the halftone dots better known as optical dot gain. The optical gain in these proofing systems give a reasonable reproduction of the tonal curves for printing presses having dot gains in the 20-30% range. The different dot gains can be obtained by increasing the coating weights of the barrier and/or adhesive layers. U.S. Pat. No. 4,262,071 describes the incorporation of a spacer layer in order to increase the optical dot gain. All of these methods are effective for achieving the higher dot gains. However, it is very difficult to achieve the dot gains in the 14-20% range. EP No. 0,243,932 describes an improved positive working color proofing film and process which claims to be able to reproduce dot gains in this lower range. This system is based upon toning a photopolymer image. Being a toning process it suffers from a lack of reproducibility of the density and lot to lot color variance of the toners. The system described in the EPO No. 243,932 patent application also incorporates optical brighteners which causes metamerism in the final proof.